How to stop my website being hijacked?

I have built a website using Wordpress at Someone is attaching spam messages by linking words in my text, i.e. run your cursor over the word and you are offered a bonus on your first gambling deposit by "CouponDropDown" or an opportunity to win a mobile phone. But these hacked words only appear in Firefox on my PC. My wife uses Firefox on a Mac and the same text on her computer is not hacked. I have run a full Norton scan and it says my system is clean. How can I stop this hacking of my website? Otherwise I may have to change browsers.

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